This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application No. 2001-048251 filed on Feb. 23, 2001, the entire content of which is incorporated herein by reference.
This invention generally relates to an electric motor control device for controlling an electric current to be supplied to an electric motor based on H∞control theory.
Switched reluctance motors (SR Motors) are conventionally employed as a driving source of electric vehicles. A conventional control device for controlling the switched reluctance motor is shown in FIG. 19. In the control device, a required torque reqtrq determined based on an accelerator operating amount is converted to both a current command value I and an angular command value xcex8 based on a command value map. (The command value map is prepared to indicate a relation between an electric current supplied to the SR Motor and an angle location at a stator that the same directional electric current is supplied.) The controller further controls the amount of electric current corresponding to the current command value I and in schedule or timing corresponding to the angular command value xcex8, and as a result the SR Motor generates a rotational torque corresponding to the required torque reqtrq. This control system is an open-loop system as shown in FIG. 19.
The SR Motor mounted on the electric vehicle may be affected by resonance characteristics of springs used for suspension in the vehicle. The SR Motor mounted on the electric vehicle may further be affected by other resonance characteristics caused by torsional deformation of a drive shaft, tires and so on connected with the SR Motor. Especially when the required torque is quickly changed as shown in FIG. 20, a rotation speed may be pulsated. Then the pulsated rotation speed appears to be a vibration in longitudial directions of the electric vehicle body. The vibration of the vehicle body may cause discomfort to the vehicle""s passengers.
To overcome the above drawback, a control method for correcting the current command value I and the angular command value xcex8 by employing a proportional derivative control (PD control) can be useful. According to this art, the vibration of the electric vehicle body can be reduced by determining a larger proportional value and a larger derivative value.
But, if the proportional value and the derivative value are determined to be large in the above PD control, noise in the motor rotation speed, which is detected by a motor rotation speed sensor, is also amplified. Thus the vibration due to the noise may be generated in the SR Motor. Moreover, stability of the control system may be spoiled. In consideration of the noise amplification and the stability of the control system, the proportional value and the derivative value are limited in size, thus the PD control cannot reduce the vibration of the electric vehicle body as sufficiently as expected. Moreover, when attempting to reduce the vibration using the above-mentioned PD control, the gain of the control system may also be reduced in all frequency domain, thus the tracking performance of the actual torque of the electric motor relative to the required torque in the control system may be spoiled. Furthermore, the acceleration performance of the electric vehicle is reduced. The above problems may also occur to an SR Motor employed as a driving source in another mechanism other than the electric vehicle.
The object of the present invention is to provide an electric motor control device having a vibration reduction performance (the performance for reducing the vibration caused by a resonance of the electric vehicle body with the electric motor), an acceleration, a noise resistance, a robust stability, and so on that are compatible with each other at a high level.
According to a first aspect of the present invention, the electric motor control device determines a required torque in response to an operation amount for an electric motor. Then the controller, in the electric motor control device, calculates a correction amount based on an actual rotation speed of the electric motor detected by a rotation speed detecting device. Then the electric motor control device controls an electric current to be supplied to the electric motor depending on a command torque obtained by correcting the required torque with a correction amount calculated by a controller. For the electric motor control device, a reference model and an actual model have been previously prepared. The reference model indicates an ideal response of the electric motor having ideal tracking performance and vibration reduction performance relative to the required torque, and the actual model indicating an actual response of the electric motor. Then the required torque is regarded as an input amount to be input into the controller. While, a revolution speed difference is determined based on the difference between a first rotation speed, calculated by inputting the required torque to the reference model, and a second rotation speed, calculated by inputting the required torque to the actual model. The rotation speed difference is regarded as a controlled amount outputted from the controller. The controller is determined as an H∞norm of a transfer function between the input amount and the controlled amount to be smaller than a predetermined value. The electric motor control device controls the electric current to be supplied to the electric motor by using the determined controller.
In the above electric motor control device, the controller calculates the correction amount based on the actual rotation speed of the electric motor detected by the rotation speed detecting device. Then the controller corrects the required torque determined in response to the operation amount of the electric motor by the above correction amount. To obtain the controller, following processes are executed. First, the reference model and the actual model are prepared. The reference model has the ideal response of the electric motor to obtain good tracking performance and the vibration reduction performance relative to the required torque. The actual model indicates the actual response of the electric motor. Next, the required torque is regarded as the input amount to be inputted to the controller. The difference between the first rotation speed, calculated by inputting the required torque to the reference model, and the second rotation speed calculated by inputting the required torque to the actual model, is regarded as the controlled amount outputted from the controller. Then the H∞control problem is solved by determining the H∞norm of the transfer function between the input amount and the controlled amount is determined to be smaller than the predetermined value.
Thus the present invention has the following advantages. Since the above controller is mounted on the electric motor control device, the actual response of the electric motor is close to the reference model which has excellent tracking performance and vibration reduction performance. Accordingly, the electric motor can be operated with better tracking performance and vibration resistance.
According to a second aspect of the present invention, in the above electric motor control device, the controller is preferably determined based on a control system in which the required torque is inputted to both the reference model and the actual model through the first weight function.
By the above technical method, since the gain of the first weight function in a particular frequency domain is determined to be large, the response of the electric motor controlled by the controller in the same particular frequency domain is closer to the reference model.
According to a third aspect of the present invention, in the above electric motor control device, the gain of the first weight function in the low frequency domain is preferably determined to be larger than the gain in the high frequency domain.
By the above technical method, the actual response of the electric motor in the low frequency domain can be made to be closer to the reference model, thus the tracking performance of the electric motor response relative to the required torque variation can be improved.
According to a fourth aspect of the present invention, in each of the above-mentioned electric motor control devices, the gain of the reference model is preferably determined to be smaller than the gain of the actual model in a frequency domain near a resonance frequency of the actual model. The gain of the reference model is determined to be approximately equal to the gain of the actual model in the remaining frequency domain.
By determining the gain of the reference model as described, a controller (i.e. the electric motor control device) capable of suppressing the vibration of the electric motor caused by a resonance can be produced.
According to a fifth aspect of the present invention, in any one of the above electric motor control devices, the actual response variation of the electric motor is calculated by a multiplicative perturbation. First, the perturbative output outputted by the multiplicative perturbation is inputted to the actual model through the first weight function. And, a second controlled amount is outputted from the controller through a second weight function. Next, the required torque and the perturbative output are regarded as the input amount, and the rotation speed difference and the second controlled amount are set as the controlled amount. Thus the controller is determined as the H∞norm of the transfer function between the above input amount and the above controlled output to be smaller than the predetermined value. Preferably, the controller controls the electric current to be supplied to the electric motor.
According to an examination, it has been proved that transfer characteristics between the required torque and the rotation speed are varied depending on driving conditions and types of vehicles (or types of machines that the electric motor is employed as a driving source). And, it has also been proved that when electric currents supplied to the motor coils are switched at high frequencies, a torque ripple is generated, and then the torque ripple negatively influences the rotation speed.
To cope with the above drawbacks, a factor bringing the actual response variation of the electric motor is described by the multiplicative perturbation. For example, the torque ripple content is described by the second weight function, then the perturbative output is outputted by the multiplicative perturbation, and added to the command torque through the second weight function. An actual transfer characteristic variation of the electric motor, depending on the varying driving conditions and the difference of vehicle types, is described by the third weight function. Thus the actual transfer characteristic variation is obtained as the second controlled amount processed through the third weight function. By the above technical method, the controller suppressing the effect by the actual response of the electric motor caused by the torque ripple and the transfer characteristic variation caused by the varying driving conditions, i.e. the electric motor control device can be produced.
According to a sixth aspect of the present invention, in the above electric motor control device, the gain of the second weight function is preferably determined to be large in the high frequency domain, and the gain of the third weight function is determined to be large in the low frequency domain.
By the above technical method, the second weight function well describes the torque ripple being large in the high frequency domain. While, the third weight function describes the transfer characteristic variation depending on the varying diving condition being large in the low frequency domain.
The actual motor rotation speed is detected by the rotation speed detecting means. Generally, the detected rotation speed contains the sensor noise (white noise), the influence of the sensor noise to the control system has to be eliminated. Moreover, the rotation speed of the electric motor corresponds to a definite integration from a time before the predetermined time of a motor torque generated by the electric motor depending on time. Even if the rotational torque is zero, the motor rotation speed is not immediately changed to be zero but to keep a constant speed value. The constant speed value is called a steady-state component of the motor rotation speed. On the other hand, the actual model is designed to be linear near the resonance frequency. Thus, the steady-state component has to be eliminated for the control performance of the controller to be improved.
To avoid the sensor noise and the steady-state component, included in the rotation speed of the electric motor, the following process is executed. According to a seventh aspect of the present invention, first, the sensor noise and the steady-state component are described by a fourth weight function relative to the sensor noise. Next, the required torque, the perturbative output and the sensor noise are regarded as the input amount. And, the controlled amount and the second controlled amount are set as the controlled amount of the controller. The controller is preferably determined to have the H∞norm being smaller than the predetermined value. By using the controller determined as above, an electric motor control device having improved robust stability can be produced.
In the above-mentioned electric motor control device, the gain of the weight function wn in the low frequency domain is preferably determined to be larger than in the high frequency domain.
The reason why the gain of the fourth weight function is as described above is because the steady-state component of the rotation speed of the electric motor, which affects the system more than the sensor noise, becomes large in the low frequency domain. By determining the fourth weight function as described above, a controller capable of more effectively stabilizing the response of the electric motor (i.e. the electric motor control device) can be produced.